1. Field of the Invention
The present invention relates to a novel powder coating material in which rate of the curing reaction can be easily controlled, from which coating layers having excellent appearance and excellent physical properties can be obtained under a desired condition in accordance with the object and which is particularly useful as a powder coating material for precoat steel plates requiring curing by baking at a high temperature in a short time. The present invention also relates to a novel precoat steel plate prepared by utilizing the powder coating composition described above.
2. Description of the Prior Art
Coating materials for precoat steel plates are generally cured at a high temperature, such as 190.degree. C. or above, preferably 210.degree. C. or above, in a short time, such as 10 minutes or less, preferably 3 minutes or less. Polyester resin powder coating materials having excellent workability are utilized for this application. For curing the coating material in a short time, catalysts, such as organotin compounds, are utilized (K. C. Frish et al., J. Macromol. Sci.-Reviews, Macromol. Chem., C5(1), 103-149 (1970)).
However, because the organotin compounds are catalysts of the reaction, it is difficult to adjust the rate of the reaction to conform with a desired condition though the reaction can be accelerated. For example, when the rate of reaction is increased, gas generated from the formed coating layer during the curing and air contained between particles of the powder coating material remain in the coating material without being completely eliminated to cause formation of blisters and pin holes in the coating layer. When the organotin catalyst is not utilized, the reaction is too slow and the coating layer having satisfactory properties cannot be obtained. These phenomena are particularly remarkably observed when a coating is cured at a higher temperature in a short time. The reaction cannot be controlled to the desired condition by using an organotin catalyst alone. For preventing the formation of blisters in the coating, various methods, such as utilization of resins having low melt viscosity, addition of thermoplastic resins and the like compounds which do not contribute to the reaction and formation of thinner coating layers, have been proposed.
However, these proposed methods have a problem that the curing reaction cannot be controlled satisfactorily and the sufficient crosslinking cannot be achieved either.